Electrochemical sensors are used in many different ways to detect chemical components in fluid media, for example, in gases.
Due to their principle of action, electrochemical sensors contain a plurality of electrodes, which communicate with one another via an electrolyte. The most common devices are amperometric sensors and comprise a working electrode and a counterelectrode or a working electrode and a counterelectrode, which is complemented by a reference electrode. The working electrode is also occasionally called a measuring electrode, and the counterelectrode is occasionally called an auxiliary electrode.
These electrochemical sensors normally comprise, a large number of individual components, which are arranged piece by piece in usually injection-molded plastic housings in a large number of steps. Present are, among other things, the aforementioned electrodes, which are mostly completely surrounded by electrolyte. In this conventional design a relatively large equalizing volume must be provided for wetting all electrodes over the broadest possible range of ambient humidities, taking into account the hygroscopic properties of most electrolytes and preventing the sensor from bursting or drying out. Minimum sizes of conventional sensor designs, which can affect a decision for or against their use, follow from the requirements imposed on the electrolyte volume as well as other design constraints.
Most electrochemical sensors have such a design that the interior space of the sensor is closed against the environment of the sensor. The access of gases or substances to be detected, which is necessary for the desired detection reaction, normally takes place through gas-permeable membranes. These gas-permeable membranes consist mostly of hydrophobic, fine-porous polymers, which retain, for example, an aqueous electrolyte in the interior space of the sensor, but offer a lower resistance to gases entering by diffusion. As an alternative, the diffusion of substances to be detected also takes place by volume diffusion through closed, nonporous polymer films. However, this method is used only for the measurement of substances that occur at a high concentration (e.g., O2 in the atmosphere). The substances to be detected must reach the working electrode in any case. The working electrode is normally arranged directly on the membrane intended for the diffusive entry of the substances to be detected or is arranged at a closely spaced location from same in the electrolyte space.
The principle of diffusion of the substance to be detected through a membrane has a number of drawbacks. Open-pore membranes have a very large surface. If such membranes are used to define the electrolyte space, adsorption phenomena may adversely affect the properties of the sensor. Open-pore and nearly open-pore membranes limit the diffusion in any case, i.e., they limit the access of gas to the working electrode and as a result reduce the sensitivity of an electrochemical sensor. This reduction in sensitivity emerges greatly in case of the use of nonporous membranes.
Moreover sealing of the interior electrolyte space of the sensor may lead to pressure differences against the environment of the sensor and often requires additional design measures for stabilizing the sensor housing. Another drawback is the strong temperature dependence of diffusion processes. If the substance to be detected reaches the working electrode by diffusion through a membrane, the sensitivity of the electrochemical sensor itself becomes temperature-dependent. A complicated temperature compensation may thus become necessary.
Open-pore membranes are, moreover, usually sensitive to rapid changes in pressure because these often lead to penetration of the electrolyte into the pores of the membrane. This is frequently followed by failure of the sensor affected.
If the working electrode is in direct contact with the permeation membrane, it is often difficult to bring about a connection between the working electrode and the membrane that is stable over the long term. However, this is necessary because the diffusion behavior of the substance to be detected is affected by changes in this contact, i.e., through penetrating electrolyte or minimum changes in distance between electrode and membrane. In particular, an electrolyte film forming between the membrane and the working electrode acts as an additional and difficult-to-calculate diffusion barrier.
The sensitivity of electrochemical sensors is determined, in addition, by the size of the effective electrode surface.
It is known that it is possible to abstain from delimiting membranes within electrochemical sensors (GB 1,552,620). As a result, there is a great loss of electrolyte in the device described there. This loss is compensated by a refilling device. However, such a refilling device means increased technical effort. In addition, the signal transduced by such a sensor depends strongly on the degree of wetting of the measuring electrode. Frequent calibrations become necessary as a result.
It is known, furthermore, that ionic liquids can be used as novel electrolyte substances in electrochemical sensors.
Ionic liquids, which are also called molten salts, have acquired increasing significance in electrochemistry in recent years because of their increasing stability to atmospheric conditions (moisture, O2) and are known as an electrolyte or an electrolyte component (DE 10245337 A1, JP2003172723 and US 20040033414 A1). The use of such liquids in membrane-covered or encapsulated electrochemical sensors is described in these documents. As a result, the sensors described have all the drawbacks that are associated with increased resistance to entry of the target gas. However, decreasing maximum allowable workplace concentration values and new measuring tasks do require electrochemical sensors with markedly improved sensitivity.